Method of chrome-retanning leather

ABSTRACT

An improved process of chrome-retanning leather in which leather to be chrome-retanned is contacted with basic chromium sulfate and a sulfite salt in an aqueous environment and thereafter the leather is contacted with an aldehyde to deposit chrome as Cr2O3 in the leather and form a leather having over 6 percent by weight of Cr2O3 based on the dry weight of the leather. Sodium sulfite or potassium sulfite can be used as the sulfite salt. Representative aldehydes which can be used are formaldehyde, glutaraldehyde, benzaldehyde, or a compound which in the presence of an acid is converted to an aldehyde.

United States Patent [1 1 Dawson June 10, 1975 METHOD OFCHROME-RETANNING LEATHER [75] Inventor: William Orval Dawson, Milwaukee,

[52] U.S. Cl. 8/94.27; 8/94.21; 8-/94.26; 8/94.33 [51] Int. Cl. Cl4c3/06 [58] Field of Search 8/94.21, 94.26, 94.27, 8/94.33

[56] References Cited UNITED STATES PATENTS 528,162 lO/l894 Heinzerling8/94.27 2,997,363 8/1961 Pfirrmann 8/94.26 3,254,937 6/1966 Komarek eta1. 8/94.26 3,346,323 10/1967 Spahrkas et al. 8/94.26 3,482,925 12/1969Schmid et al. 8/94.17 3,519,378 7/1970 Luck et al 8/94.26 3,551,089l2/l970 Dawson et al. 8/94.27 3,558,262 l/l97l Duffy et al 8/94.27

OTHER PUBLICATIONS Wilson, The Chemistry of Leather Manufacture, 1929,

Vol. II ACS TS965W5, p. 611.

Gustavson The Chemistry of Tanning Processes 1956, pp. 339-40, TS967G8.

Primary ExaminerBenjamin R. Padgett Assistant ExaminerB. Hunt Attorney,Agent, or Firm-Merriam, Marshall, Shapiro & Klose [5 7 ABSTRACT Animproved process of chrome-retanning leather in which leather to bechrome-retanned is contacted with basic chromium sulfate and a sulfitesalt in an aqueous environment and thereafter the leather is contactedwith an aldehyde to deposit chrome as Cr O in the leather and form aleather having over 6 percent by weight of Cr O based on the dry weightof the leather.

Sodium sulfite or potassium sulfite can be used as the sulfite salt.Representative aldehydes which can be used are formaldehyde,glutaraldehyde, benzaldehyde, or a compound which in the presence of anacid is converted to an aldehyde.

27 Claims, N0 Drawings METHOD OF CHROME-RETANNING LEATHER This inventionrelates to processes for producing leather from animal hides. Moreparticularly, this invention is concerned with an improved process ofproducing leather by chrome-retanning.

Large quantities of animal hides are converted into leather. Althoughcattle hides are the most important raw material for tanners inproducing leather, there is a considerable production of leather fromcalf, sheep, hog and goat hides.

The conversion of an animal hide into leather is done by tanning.Although tanning can be effected by using essentially vegetablematerials, and particularly materials which contain appreciable amountsof tannin, such as the bark and wood of oak, chestnut and wattle,probably a considerably greater amount of leather is .produced by theprocesss of chrome-tanning.

Irrespective of the tanning process used, however, the animal hides areusually dehaired. One method of dehairing is to soak the hide in a limesolution containing additives designed to aid in hair removal and tothen mechanically remove the hair. The dehaired hides, from which allflesh is removed by scraping, are then treated in a rotating drum with adeliming material such as ammonium chloride or ammonium sulfate toneutralize the lime. A pancreatic enzyme is added to the delimingsolution to aid in bating and deliming and to further condition thehides. After being washed with water, the hides, after draining, areready for the tanning process.

In the first step of a conventional chrome-tanning operation, the hidesare pickled with a solution of an acid, usually sulfuric acid, andsodium chloride. A chrometanning solution prepared by reducing sodiumbichromate in a conventional manner is then introduced into the drum.The drum is rotated during the tanning operation. The basicity of thechrome-tanning solution can vary consideraly. Normally, it will be inthe range of about 25 to 50 percent. Furthermore, the conditions ofchrome-tanning, such as the volume of the chrometanning liquor, theconcentration of chromium salts and other ingredients in the liquor andthe treatment time are all matters that are well known by those skilledin the art.

Following such a conventional chrome-tanning operation, the hides aresorted according to the use to which the leather ultimately will be put.Subsequently, at least in the United States, the chrome-tanned leatheris split. In Europe, however, the leather generally is split after thehair is removed from the hide and before it is bated, pickled andchrome-tanned.

In conventional methods of leather production, the chrome-tanned leatherin split form is then treated according to a wide variety of processesto obtain desirable characteristics such as softness, firmness,uniformity of temper, smoothness, grain break and color. Then theleather is fat-liquored, which is essentially the introduction ofvarious water dispersible fats and oils into the leather to preserve andsoften the leather.

Although the described general process for producing chrome-tannedleather is suitable for many types of leather production, it is oftenfound that the resulting product lacks the desired softness, fullnessand chrome content. Accordingly, to produce leather with these desirableproperties and to raise the chrome content, it is common to subject thechrome-tanned leather to a chrome-retanning process.

In conventional chrome-retanning, the chromeretanning liquors used aregenerally similar to those used in the initial chrome-tanning process.Chromeretanning as presently practiced is a time-consuming operation andgenerally leaves considerable to be desired, both in the process and theresulting leather. Most of the time the operation leads to results farshort of the desirable fullness, softness and increased chrome contentsought in the leather. Furthermore, resistance to shrinkage in thepresence of heat, and resistance to perspiration, generally are notimproved substantially using conventional chrome-retanning processes.Very seldom can the chrome content of the leather on a dry basis beincreased 2 to 4 percent of Cr O by a chromeretanning operation. Sincethe initial Cr O content of the chrome-tanned leather on a moisture freebasis is in the neighborhood of about 2.5 to 3.5 percent, it isgenerally not possible by conventional chromeretanning operations toproduce leather having much over 5.5 percent Cr O on a moisture freebasis.

Dawson et al. US. Pat. No. 3,551,089 discloses a chrome-retanning methodusing ammonium zirconyl carbonate. This procedure raises the chromecontent of a chrome-retanned leather from a high of about 3.5 percent toover 4.5 percent and up to nearly 6 percent. However, higher amountsthan 6 percent of chrome in the leather are desired so as to improve theheat stability of the leather and to generally improve its properties.

It has been found according to the present invention that improvedchrome-retanned leathers can be produced by subjecting an animal hide,previously initially chrome-tanned by conventional procedures, to theaction of a basic chrome-retanning solution and a sulfite salt followedby the addition of an aldehyde to the solution while in contact with theleather. The process results in deposition of chrome calculated as Cr Oin the leather in amounts from 50 to 200 percent higher than is achievedby conventional chrome-retanning procedures.

Treatment of a chrome-tanned leather with a basic chrome-retanningsolution followed by treatment with an aldehyde does not effectchrome-retanning of the leather because the chrome is not precipitatedfrom solution unless the pH is raised and if not precipitated it doesnot combine with the leather and hence it will be readily removed duringwashing or other chemical applications. However, if the pH is raised byadding a base, such as sodium hydroxide, to the solution, the Cr O isprecipitated on the leather and in the solution, rather than in theleather.

A basic chrome-retanning solution containing a sulfite salt is notsuitable for effecting chrome-retanning without an aldehyde because thesulfite salt raises the pH and the basicity of the chromium sulfatesolution, solubilizes the chrome and masks the solution so thatprecipitation of chrome does not occur, even at the higher pH. The sameresult occurs when a chrometanned leather is treated with a basicchrome-retanning solution followed by treatment of the leather with asolution of a sulfite salt. However, if such a leather treatment isfollowed by treatment of the leather with an aldehyde while the chromeand sulfite are in contact with the leather, the masking or complexingeffect exerted by the sulfite on the chrome in solution is overcome bythe aldehyde. The aldehyde causes the pH to rise quite quickly and thechrome to precipitate and deposit in the leather, with full exhaustionof the chrome from solution.

The process of the invention can be practiced using any conventionalchrome-retanning solution containing basic chromium sulfate. Thebasicity of such solutions can vary widely but usually will be about 25to 60 percent. A chrome-retanning solution can be used having a volumeand containing an equivalent ofCr O sufficient to supply enough chromeso that the chrome increase, calculated as Cr O in the leather will beto the extent desired but, of course, within the limits of theinvention. Normally, the chrome-retanning solution used should containsufficient Cr O to provide about 1 to 6 percent by weight of Cr O basedon the dry weight of the leather of additional Cr O above the amountdeposited in the leather by conventional, initial chrometanning. Thechrome solution will generally have a pH of about 4.5 to 3.7 or lower.

Any suitable sulfite salt which is soluble under the conditions used inthe process can be employed. The alkali metal sulfite salts arepresently considered most suitable. Sodium sulfite and potassium sulfiteare par ticularly suitable since they are readily available. Whilecalcium sulfite and other alkaline earth metal sulfite salts such asmagnesium sulfite can be used, as well as ammonium sulfite, these saltsare not considered as suitable as the alkali metal salts.

Although the amount by weight of Cr O in the chrome-retanning solutionwith respect to the amount by weight of sulfite salt used in the processis not narrowly critical, it is generally advisable to employ thesematerials in a ratio of about 1 to 4 parts by weight of sulfite salt perone part by weight of Cr O equivalent in the solution. On a molecularweight basis, the equivalent of about one mole of Cr O should be presentfor each 2 to 4 moles of sulfite salt.

It presently appears that any aldehyde can be used in practicing thechrome-retanning process of this invention. Thus, straight and branchedchain hydrocarbon monoaldehydes such as formaldehyde, acetaldehyde andcaproaldehyde; aryl aldehydes such as benzaldehyde and l-naphthaldehyde;polyaldehydes such as glyoxal, malonaldehyde, glutaraldehyde andphthalaldehyde; dialdehyde starch; and compounds which convert toaldehydes readily, can be used in the process. A series of commercialcompounds marketed under the trade name Oxazolidine are particularlyuseful in the process even though they are not aldehydes because in thepresence of an acid they are converted to aldehydes. SpecificOxazolidines which can be used in this way are Oxazolidine T of theformula cn on O N O and having the chemical name of5-hydroxymethyl-laza-3,7-dioxabicyclo[3,3,0]octane;

Oxazolidine E of the formula and having the chemical name of5-ethyl-l-aza-3,7- dioxabicyclo [3,3,0]octane; and

Oxazolidine A of the formula HN O and having the chemical name of4,4-dimethyl-l ,3- oxazoline. These compounds, and others of similarstructure, in the presence of an acid such as sulfuric acid,hydrochloric acid, phosphoric acid and acetic acid, or an acid salt suchas sodium acid sulfate or monosodium dihydrophosphate, converts to analdehyde by ring cleavage. The resulting aldehyde functions suitably inthe chrome-retanning process of this invention.

The amount of aldehyde used in the process is not especially critical.However, enough aldehyde is to be used to react with all of the sulfitesalt to form a basic byproduct of the reaction according to thereaction:

wherein R is hydrogen or an organic residue and M is a metallic ion. Thealdehyde combines with the sulfite by omega sulfonation to release abase, such as sodium hydroxide when M is Na. The basic by-product MOH,and advisably sodium hydroxide, raises the pH of the chrome-retanningsolution and thereby causes the chrome to deposit in the leather. It isgenerally advisable to use about 0.1 to 2 parts by weight of an aldehydeper each part by weight of sulfite salt used in the treatment. It shouldbe understood however that the best ratio or proportion to use is to bedetermined for the specific materials employed using the normal skill inthe art. It is to be expected that the ratios may vary from one aldehydeto another according to the reactivity of the aldehyde and whether it isa monoaldehyde or polyaldehyde.

The ratio of equivalents of Cr O used in the process to the amount ofaldehyde can vary quite widely but usually will be in the ratio of 1part by weight of Cr O to 0.25 to 3 parts by weight of an aldehyde on apercent aldehyde basis.

It should be understood with respect to all of the ra tios presentedherein that the use of specific materials may require the use of amountsoutside of the stated typical ratios. Nevertheless, the use of suchratios and materials is to be included within the scope of theinvention.

It is advisable in practicing the invention to first bring thechrome-tanned leather which is to be chromeretanned into contact with abasic solution of chromium sulfate. The leather, for example, can beplaced in a conventional tanning drum and a basic chromeretanningsolution, devoid of a sulfite salt and an aldehyde, can be addedfollowed by the addition of the sulfite salt and then by the addition ofthe aldehyde. After the separate addition of each material in thedescribed order the drum can be rotated to bring the materials intointimate contact with all areas of the leather.

Although it is advisable to first treat the chrometanned leather with abasic chrome-retanning solution containing no sulfite salt and noaldehyde, it is feasible to treat the leather with a basicchrome-retanning solution containing a sulfite salt but no aldehyde. Thesulfite salt can be added to a basic chrome-retanning solution and thesolution then added to a leather processing drum containing the leatherto be chromeretanned. Although the sulfite salt slowly causes the pH ofthe solution to increase, i.e., become more basic, the chrome does notprecipitate out of the solution until after the aldehyde is added. Theaddition of the aldehyde results in precipitation of the chrome into theleather, with essentially complete exhaustion of the chrome from thesolution and production of the desired chromeretanned leather.

The chrome-retanning process can be effected at any temperature foundsuitable for the conditions and equipment although generally thesolutions should be employed at a temperature of about 70 to 130F. Thetime of treatment after all the ingredients used in the process havebeen brought in contact with the leather is not considered narrowlycritical. The treatment can be continued for such time as is necessaryto effect deposit of the chrome in the leather. Under normal conditionsthis will take from about minutes to 1 hour to completely exhaust thechrome from the solution. It should be understood that the total amountof chrome in solution should not be larger than the amount of chromewhich the leather can take up since the rest would be wasted. When theprocess is practiced properly all the chrome used will be exhausted fromsolution and be taken up by the leather.

Leathers that are chrome-retanned according to the invention have chromecontents based on Cr O of more than 4 percent by weight of dry leatherand generally will contain 8 to 9 percent or more of Cr O depending uponthe amount desired in the leather. Such large amounts of chrome aredeposited in the leather without deleterious effects on the leather and,to the contrary, with the production of leathers having superiorproperties.

After the chrome-retanning is completed, the leather can be furtherprocessed using conventional procedures of coloring and fat-liquoring.The leather so produced has excellent properties, is more heat stablethan other leathers because of its high chrome content, has goodfullness, is soft and is tight breaking.

EXAMPLE 1 Eight hundred pounds of chrome-tanned split and shaved cowhides (8O sides) containing 3 percent by weight of chrome calculated asCr O on the dry weight of the leather was placed in a rotating drum andfloated with 192 gals. (U.S.) of water at 115F. The water equalled 200percent by weight based on the weight of split and shaved leathercontaining about 50 percent by weight of water. The leather was washedover a 20 minute period using 840 gals. (U.S.) of water at 115F.

The drum was drained of water and then the leather was floated in 72gals. (U.S.) of water at 115F. To the leather was then added 19.2 gals.(U.S.) ofa 42 percent basic chrome liquor at 115F. calculated to provide3 percent by weight of chrome as Cr- O to the leather on a wet leatherbasis. The leather was drummed for 15 minutes and then pounds ofanhydrous sodium sulfite was added followed by 45 minutes of drumming.The solution had a pH of 5.75.

Eighty pounds ofa 16 percent by weight sulfuric acid solution in 14.4gals. (U.S.) of water was added to 80 pounds of a 50 percent aqueoussolution of 5-ethyl-1- aza-3,7-dioxabicyclo [3,3,0]octane sold asOxazolidine E. The resulting mixture was added to the leather in thedrum, The leather was drummed for 45 minutes. Complete exhaustion ofchrome into the leather from the solution was achieved. On a dry leatherbasis the leather therefore theoretically contained at least 9 per centby weight of chrome calculated as Cr O The pH of the solution in thedrum was 5.9.

Although the chrome-retanning according to the invention is complete atthis point, additional treatments of the leather are performed accordingto conventional procedures to obtain a commercial, finished leather. Thefollowing procedure is representative of those which can be used tofurther process the chromeretanned leather.

The chrome-retanned leather was washed in the drum for 20 minutes using840 gals. (U.S.) of water at F. The water was drained from the drum. Theleather was then floated in the drum in 72 gals. (U.S.) of water at120F,

To the drummed leather was added 40 pounds of an anionic aminoplastresin (Chemtan R-9) in 28.6 gals. (U.S.) of water at 120F. The leatherwas drummed 10 minutes and then 16 pounds of dry boric acid was added.The leather was then drummed for 20 minutes.

The leather was then dyed using 3 pounds of yellow dye (Ortolan YellowRR), 1 pound of beige dye (Lurazol Beige), and 1 pound of brown dye(Luganil Brown N36). The dyes were added to 24 gals. (U.S.) of water,the solution was boiled, cooled to 120F. and then added to the drummedleather. The leather was drummed for 20 minutes.

To the leather was added 48 pounds of blended vegetable tannins and 6.4gals. (U.S.) of a 42 percent basic chrome solution calculated to add 1percent by weight of chrome as Cr O to the leather in 28.6 gals. (U.S.)of water at 120F. The leather was drummed for 30 minutes. The pH of thesolution was 4.2.

Two pounds of formic acid in 9.6 gals (U.S.) of water at 80F. was addedto the leather to set the dye. The leather was washed 20 minutes withwater at F. The leather was drained of water and then floated in 96gals. (U.S.) of water at 130F. To the leather was then added 44 poundsof a sulfated mixture of vegetable, fish and animal oils in 24 gals.(U.S.) of water at 130F. The leather was then drummed for 45 minutes.

To the leather was added 20 pounds of a cationic emulsified raw oil in19.4 gals. (U.S.) of water at 130F. as a topping oil. The leather wasdrummed 15 minutes. The solution pH was 4.2. The leather was thenremoved from the drum, horse smoothed and dried.

A st'ratographic analysis was made for chrome calculated as Cr O in theleather. The results of this process are given in Table 1 together withcomparative data for a conventional chrome-retanned leather and for aregular nonchrome-retanned leather.

Table 1 Percent C r on a dry basis Example 1 Conventional Regularchromechromenonchromeretanned retanned retanned Grain slice 1 1.7 5.42.9 Second slice 10.8 5.8 3.5 Third slice 8.1 5.8 3.4 Fourth slice 9.45.8 3.2 Flesh slice 10.7 5.4 2.6

EXAMPLE 2 A IOO-pound group of nine chrome-tanned split sides shaved to4.25 to 4.75 oz. (1 oz. equals 0.4 mm. of leather thickness) containing3 percent Cr O based on the leather, dry weight, were placed in a drumand washed 10 minutes at 115F. with water and drained. The leather wasfloated in 7.5 gals. (U.S.) (50 percent by weight) of water at 1 15F.and then 3 percent Cr O based on the leather, wet weight, as a 42percent basic chrome liquor was added at 1 15F. and rotated for 15minutes. The pH of the solution in the drum was 3.4. Then to the leatherwas added 10 percent by weight based on the wet weight of the leather,of anhydrous sodium sulfite following which the drum was rotated for 60minutes.

An amount of sulfuric acid equal to 1.6 percent by weight of the wetleather was diluted with an amount of water equal to 10 percent byweight of the wet leather at 80F. The resulting solution was added to anamount of a 50 percent solution of 5-ethyl-l-aza-3,7-dioxabicyclo[3,3,0]octane, sold under the trade name Oxazolidine E, inwater equal to percent of the weight of the wet leather. The drum wasthen rotated for 45 minutes. The pH of the solution in the drum wentfrom 5.2 to about 6.0 to 6.5. The chrome was completely exhausted.

The leather was then washed 5 minutes at 120F., drained and floated in50 percent by weight of water at 120F. The leather was then finishedaccording to conventional procedures. It contained 9 percent by weightof Cr O on a dry leather basis.

EXAMPLE 3 The procedure of Example 2 is followed using an equal amountof glutaraldehyde in place of Oxazolidine E and without use of thesulfuric acid. The leather obtained has a high chrome content which mayaverage 9 percent by weight as Cr O EXAMPLE 4 The procedure of Example 1is followed using an equal amount of formaldehyde in place ofOxazolidine E and without use of the sulfuric acid. The leather obtainedhas a chrome content of at least 9 percent by weight as Cr O Theforegoing detailed description has been given for clearness ofunderstanding only, and no unnecessary limitations should be understoodtherefrom, as modifcations will be obvious to those skilled in the art.

What is claimed is:

1. In the process of chromeretanning leather, the improvement whichcomprises contacting a leather to be chrome-retanned with basic chromiumsulfate and a sulfite salt in an aqueous environment and thereaftercontacting the leather with an aldehyde to deposit chrome as Cr O in theleather and form a leather having over 6 percent by weight of Cr O basedon the dry weight of the leather.

2. The process of claim 1 in which the basicity of the chromium sulfatein contact with the leather is about 25 to 60 percent.

3. The process of claim 2 in which the solution is at a temperature ofabout to F during contact with the leather. 7

4. The process of claim 1 in which the sulfite salt is an alkali metalsalt.

5. The process of claim 4 in which the alkali metal salt is sodiumsulfite or potassium sulfite. I

6. The process of claim 1 in which the aldehyde is formaldehyde. 7

7. The process of claim 1 in which the aldehyde is glyoxal.

8. The process of claim 1 in which the aldehyde is glutaraldehyde.

9. The process of claim 1 in which the aldehyde is benzaldehyde.

10. The process of claim 1 in which the aldehyde is acetaldehyde.

11. The process of claim 1 in which the aldehyde is formed by acidifyinga member of the group consisting of S-hydroxymethyL l -aza-3,7-dioxabicyclo[3 ,3 ,0]octane, S-ethyl- 1 -aza-3 ,7-dioxabicyclo[3 ,3,0]octane and 4,4-dimethyl-l ,3-oxazolidine.

12. In the process of chrome-retanning leather, the improvement whichcomprises:

contacting a leather to be chrome-retanned with an aqueous solution ofbasic chromium sulfate; thereafter contacting the leather with a sulfitesalt in aqueous solution; and

thereafter contacting the leather with an aldehyde in aqueous solution,thereby depositing chrome as Cr O in the leather to form a leatherhaving over 6 percent by weight of Cr O based on the dry weight of theleather.

13. The process of claim 12 in which the basicity of the solution isabout 25 to 60 percent.

14. The process of claim 12 in which the solution is at a temperature ofabout 70 to 130F during contact with the leather.

15. The process of claim 12 in which the sulfitesalt is an alkali metalsalt. I

16. The process of claim 15 in which the alkali metal salt is sodiumsulfite or potassium sulfite.

17. A chrome-retanning solution according to claim 12 in which thesulfite salt is sodium sulfite or potassium sulfite, and the aldehyde isa member of the group consisting of formaldehyde, glyoxal,glutaraldehyde, acetaldehyde, dialdehyde starch, benzaldehyde and analdehyde formed by acidification of a member of the group consisting ofS-hydroxymethyl-l-aza-3,7-dioxabicyclo[ 3 ,3,0]octane,5-ethyl-1-aza-3,7-dioxabicyclo[ 3,3,0]octane and 4,4-dimethyl-l,3-oxazolidine.

18. The process of claim 12 in which the amount of Cr O in the leatheris increased about 1 to 6 percent by weight, by the chrome-retanningprocess, over the Cr O content of the leather before saidchromeretanning.

19. In the process of chrome-retanning leather, the improvement whichcomprises:'

contacting a leather to be chrome-retanned with an aqueous solution ofbasic chromium sulfate with said solution containing a sulfite salt; and

thereafter contacting the leather with an aldehyde in aqueous solution,

thereby depositing chrome as Cr O in the leather to form a leatherhaving over 6 percent by weight of Cr O based on the dry weight of theleather.

20. The process of claim 19 in which the basicity of the solution isabout 25 to 60 percent.

21. The process of claim 19 in which the solution is at a temperature ofabout 70 to 130F during contact with the leather.

22. The process of claim 19 in which the sulfite salt is an alkali metalsalt.

23. The process of claim 22 in which the alkali metal salt is sodiumsulfite or potassium sulfite.

24. A chrome-retanning solution according to claim 19 in which thesulfite salt is sodium sulfite or potassium sulfite, and the aldehyde isa member of the group consisting of formaldehyde, glyoxal,glutaraldehyde, acetaldehyde, dialdehyde starch, benzaldehyde and analdehyde formed by acidification of a member of the group consisting of5-hydroxymethyl-l-aza-3,7-dioxabicyclo[ 3 .3 ,Oloctane, S-ethyll -aza-3,7-dioxabicyclo[ 3,3,01octane and 4,4-dimethyl-l ,3-oxazolidine.

25. The process of claim 19 in which the amount of Cr O in the leatheris increased about 1 to 6 percent by weight, by the chrome-retanningprocess, over the Cr O content of the leather before saidchromeretanning.

26. A chrome-retanning aqueous solution comprising a sulfite salt, analdehyde and basic chromium sulfate.

27. A chrome-retanning solution according to claim 26 in which thesulfite salt is sodium sulfite or potassium sulfite, the aldehyde is amember of the group consisting of formaldehyde, glutaraldehyde,acetaldehyde, glyoxal, dialdehyde starch, benzaldehyde or an aldehydeformed by acidification of S-hydroxymethyll-aza-3 ,7-dioxabicyclo[ 3 ,3,0]octane, S-ethyll -aza- 3,7-dioxabicyclo[3,3,0]octane or4,4-dimethyl-l,3-

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTIONPATENT NO. 1 3,888,625

DATED 3 June 10, 1975 INVENT0R(5) I William Orval Dawson It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 8, line 59, "solution" should be process; column 9, line 25,"solution" should be -process.

Signed and Scaled this fifteenth D3) of June 1976 [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner vjlaremsand Trademarks

1. IN THE PROCESS FOR CHROME-RETANNING LEATHER, THE IMPROVEMENT WHICHCOMPRISES CONTACTING A LEATHER TO BE CHROMERETANNED WITH BASIC CHROMIUMSULFATE AND A SULFITE SALT IN AN AQUEOUS ENVIRONMENT AND THEREAFTERCONTACTING THE LEATHER WITH AN ALDEHYDE TO DEPOSIT CHROME AS CR2O3 INTHE LEATHER AND FORM A LEATHER HAVING OVER 6 PERCENT BY WEIGHT OF CR2CO3BASED ON THE DRY WEIGHT OF THE LEATHER.
 2. The process of claim 1 inwhich the basicity of the chromium sulfate in contact with the leatheris about 25 to 60 percent.
 3. The process of claim 2 in which thesolution is at a temperature of about 70* to 130*F during contact withthe leather.
 4. The process of claim 1 in which the sulfite salt is analkali metal salt.
 5. The process of claim 4 in which the alkali metalsalt is sodium sulfite or potassium sulfite.
 6. The process of claim 1in which the aldehyde is formaldehyde.
 7. The process of claim 1 inwhich the aldehyde is glyoxal.
 8. The process of claim 1 in which thealdehyde is glutaraldehyde.
 9. The process of claim 1 in which thealdehyde is benzaldehyde.
 10. The process of claim 1 in which thealdehyde is acetaldehyde.
 11. The process of claim 1 in which thealdehyde is formed by acidifying a member of the group consisting of5-hydroxymethyl-1-aza-3,7-dioxabicyclo(3,3,0)octane,5-ethyl-1-aza-3,7-dioxabicyclo(3,3,0)octane and4,4-dimethyl-1,3-oxazolidine.
 12. In the process of chrome-retanningleather, the improvement which comprises: contacting a leather to bechrome-retanned with an aqueous solution of basic chromium sulfate;thereafter contacting the leather with a sulfite salt in aqueoussolution; and thereafter contacting the leather with an aldehyde inaqueous solution, thereby depositing chrome as Cr2O3 in the leather toform a leather having over 6 percent by weight of Cr2O3 based on the dryweight of the leather.
 13. The process of claim 12 in which the basicityof the solution is about 25 to 60 percent.
 14. The process of claim 12in which the solution is at a temperature of about 70* to 130*F duringcontact with the leather.
 15. The process of claim 12 in which thesulfite salt is an alkali metal salt.
 16. The process of claim 15 inwhich the alkali metal salt is sodium sulfite or potassium sulfite. 17.A chrome-retanning solution according to claim 12 in which tHe sulfitesalt is sodium sulfite or potassium sulfite, and the aldehyde is amember of the group consisting of formaldehyde, glyoxal, glutaraldehyde,acetaldehyde, dialdehyde starch, benzaldehyde and an aldehyde formed byacidification of a member of the group consisting of5-hydroxymethyl-1-aza-3,7-dioxabicyclo(3,3,0)octane,5-ethyl-1-aza-3,7-dioxabicyclo(3,3, 0)octane and4,4-dimethyl-1,3-oxazolidine.
 18. The process of claim 12 in which theamount of Cr2O3 in the leather is increased about 1 to 6 percent byweight, by the chrome-retanning process, over the Cr2O3 content of theleather before said chrome-retanning.
 19. In the process ofchrome-retanning leather, the improvement which comprises: contacting aleather to be chrome-retanned with an aqueous solution of basic chromiumsulfate with said solution containing a sulfite salt; and thereaftercontacting the leather with an aldehyde in aqueous solution, therebydepositing chrome as Cr2O3 in the leather to form a leather having over6 percent by weight of Cr2O3 based on the dry weight of the leather. 20.The process of claim 19 in which the basicity of the solution is about25 to 60 percent.
 21. The process of claim 19 in which the solution isat a temperature of about 70* to 130*F during contact with the leather.22. The process of claim 19 in which the sulfite salt is an alkali metalsalt.
 23. The process of claim 22 in which the alkali metal salt issodium sulfite or potassium sulfite.
 24. A chrome-retanning solutionaccording to claim 19 in which the sulfite salt is sodium sulfite orpotassium sulfite, and the aldehyde is a member of the group consistingof formaldehyde, glyoxal, glutaraldehyde, acetaldehyde, dialdehydestarch, benzaldehyde and an aldehyde formed by acidification of a memberof the group consisting of5-hydroxymethyl-1-aza-3,7-dioxabicyclo(3,3,0)octane,5-ethyl-1-aza-3,7-dioxabicyclo(3,3, 0)octane and4,4-dimethyl-1,3-oxazolidine.
 25. The process of claim 19 in which theamount of Cr2O3 in the leather is increased about 1 to 6 percent byweight, by the chrome-retanning process, over the Cr2O3 content of theleather before said chrome-retanning.
 26. A chrome-retanning aqueoussolution comprising a sulfite salt, an aldehyde and basic chromiumsulfate.
 27. A chrome-retanning solution according to claim 26 in whichthe sulfite salt is sodium sulfite or potassium sulfite, the aldehyde isa member of the group consisting of formaldehyde, glutaraldehyde,acetaldehyde, glyoxal, dialdehyde starch, benzaldehyde or an aldehydeformed by acidification of5-hydroxymethyl-1-aza-3,7-dioxabicyclo(3,3,0)octane,5-ethyl-1-aza-3,7-dioxabicyclo(3,3,0)octane or4,4-dimethyl-1,3-oxazolidine.